The following patents and published patent applications are believed to represent the current state of the art:
U.S. Pat. Nos. 6,404,393; 6,115,762; 6,031,505; 4,100,013; 4,242,369; 4,668,533; 6,658,314; 6,259,962; 6,582,979; 6,765,183; 6,249,261; 6,501,437; 6,575,374; 6,735,183; 6,818,985; 6,251,488; 6,636,676; 6,811,744; 6,823,124; 6,642,893; 6,037,906; 6,351,241; 5,204,687 and 5,943,020.
Published PCT Patent Application WO2004/068389.
Published U.S. Patent Applications 2004/0197493; 2004/0179808 and 2005/0046664.